Edited to add: Aaah, now you've edited your post it makes my original answers look a bit .... odd so, in the spirit of editing, I've adjusted my answers accordingly:
1. Yes.
2. Inside my rucksack until required. Then the outside of my rucksack, attached via existing bungee cords or in the mesh panel on the outside of my rucksack. *
3. It would work by preventing my wet clothes from making the rest of my stuff wet.
4. Waterproof. Compact. Lightweight.
5. Anyone wanting to keep wet clothes separate from dry clothes. No particular age range and anyone might need to do it.
* What would be useful is a rucksack made of totally waterproof material (including zips) to keep the insides dry but with a mesh panel on the outside to house wet clothes.
Edited again: Sorry jimbob, my edited answer now makes your comment below look a bit .... odd